Materials such as paints, coatings and sealants often have a limited shelf-life. After years or even months of storage, a material's viscosity can increase to a level that the material is unusable. Indeed, the material may have hardened to a point that it can no longer be removed from the can. Disposal of these highly viscous fluids is expensive because they usually must be burned to satisfy environmental protection regulations. The cost of disposal, as well as the cost of the material, could be saved if a device existed which could pump extremely viscous fluids out of their container while at the same time reprocessing them to a reusable condition.
There are also many by-products in the manufacturing of plastics which are highly viscous and present extreme problems with respect to their disposal. For example, a by-product called pyrolysis pitch is produced in the manufacture of polypropylene and polyethylene. This material is a highly viscous material having a viscosity in the range of several hundred thousand centipoise. Because of the inability to remove the material from storage pits in which it is deposited during the manufacturing process, the material has been largely unused. Where the material can be collected, because of the viscosity of the material, it is considered waste and presents a problem and expense with respect to disposal. The material includes high contents of carbon and thus a method capable of pumping and mixing requisite surfactants and other components with the material to both facilitate its removal and subsequent transformation into usable materials, such as fuels, would be extremely beneficial.
Viscous fluids are by definition resistant to flow, and this property causes unique problems when trying to pump such materials. Producing viscous fluid outputs with high volume and pressure can be accomplished by many different types of pumps, if the driving energy is sufficient and the pump materials are properly selected to withstand the forces required to achieve pumping action.
The greatest problem encountered when trying to pump viscous fluids is supplying fluids to the pump unit. The input side of any pump functions as a suction device, in that fluid is drawn into the pump, and surrounding fluid must flow toward the pump to replace fluid that is entering the pump. The greatest suction that can be produced at the pump input is zero pounds per square inch absolute pressure (psia). Trying to operate the pump under conditions to produce greater suction is futile since zero psia represents an absolute vacuum. For example, if the absolute pressure at a pump's input is nearing zero psia and the pump is driven faster, the fluid will merely separate and a vacuum void or cavity will be produced at the input. This input cavitation effect prevents the pump from producing any greater output volume.
Since atmospheric pressure is approximately 14.7 psia and zero psia is the lowest obtainable pressure at the pump input, 14.7 pounds per square inch (psi) is the maximum differential pressure available to force the fluid toward the pump inlet to replace fluid that is being drawn into the pump. This limiting factor of 14.7 psi maximum differential pressure between atmospheric pressure and pump inlet pressure causes very few problems with low viscosity fluids. Because these fluids exhibit relatively low resistance to movement, the available pressure differential is sufficient to allow fluid to flow toward the pump inlet at the same rate as fluid is entering the pump. Only when operating a pump of relatively low volumetric capacity at relatively high cyclic rates do input voids cavitation) present a problem when pumping low viscosity fluids. High viscosity fluids can present a severe problem, however, particularly if large volumes of flow are required. This is because at the maximum available differential pressure, the rate of flow toward the pump inlet is relatively slow due to the high resistance to flow exhibited by the viscous fluids.
Current designs of high volume pumping systems for viscous fluids utilize piston pumps in various configurations. The greatest disadvantage of piston pumps is that while considerable energy is required to maintain the flow of viscous fluids, even greater energy is required to initiate flow due to the requirements of overcoming the inertia of the material. With the conventional piston pump system, the fluid is moved only on one-half of the cycle; thus, fluid flow is initiated once per cycle and halted once per cycle. This halt in fluid flow wastes input energy and requires a relatively large power source to reinstate fluid flow during each cycle.
Conventional design gear pumps can pump viscous fluids only at relatively low volumes because near the input zone, as the gear teeth are parting from the meshed condition, fluid must flow into the void that is created by the parting gear teeth. The fluid must flow into and fill this void before the teeth move to the position where they seal against the pump housing. Viscous fluid flows into this void relatively slowly. To ensure that the pump does not cavitate, it must be initially run at very slow speeds to allow this void to fill.
Where waste or expense is a consideration, fluid pumps for pumping viscous materials will usually be employed with a platen or follower plate. The pump is usually mounted above an orifice through the platen. The platen is sealingly introduced into the shipping container where the viscous fluid is located. As liquid is evacuated, the descending unit scraps down the sides, theoretically causing all of the material to remain in the path of the pump. Additionally, the platen forces the material to the pump to prime it and to keep it primed against the effects of cavitation. Commonly, these results are achieved through the application of external forces by means of a pneumatic or hydraulic ram or mechanical spring-driven rams. The unfortunate consequences of ram-powered platens are (1) the wear and eventual failure of the circumferential seal; (2) severe limitation of portability; and (3) proliferation of systems leading to expensive maintenance and downtime.
An example of a ram-powered platen is found in U.S. Pat. No. 4,592,491 to Chollet. Chollet discloses a device for emptying recipients containing products of high viscosity. A follower plate applies pressure on the contents of a drum by means of vertical tie rods. A scraper system engages the hardened product while heat is applied to soften it. The Chollet device, however, does not reconstitute the material to usable form. Also, the Chollet device requires external force to drive the scraper into the hardened material.
A further example of a platen-pump combination is found in U.S. Pat. No. 4,635,820 to Marshall. Marshall discloses a device which unloads containers of solidified thermoplastic material by heating the material until it softens. The softened material is pumped off of the surface. Springs urge the platen further into the container.
Pumps may also be used to mix additives into viscous or non-viscous fluids. Surfactants are added to decrease the viscosity of materials. Catalysts or other chemical activators, known in the trade as "trippers", are added to a material to allow it to cure more quickly, decreasing the fluid's shelf-life. Unfortunately, trippers are added at the factory long before the sale of the material.
A need exists for a device that can pump extremely viscous materials from a drum or container. The apparatus should also be able to convert the materials to a usable viscosity range without affecting other physical properties of the materials. Such an apparatus should not require the application of any external force on the viscous fluid, nor should it require an external heat source to soften the materials first. Such a pump should also be capable of mixing materials as needed to convert the products to a usable state.